1. Field of Invention
This invention relates generally to an electric vehicle of the type having a hydraulic system for powering various hydraulic work elements employed thereon, and more particularly to means for minimizing the power drain placed on the vehicle's battery by such hydraulic system.
2. Description of the Prior Art
In order to achieve the maximum useful work time per battery charge and to minimize the operating cost of an electric vehicle, such as a lift truck, it is necessary to conserve as much energy as possible during operation. Aside from the propulsion requirements of such lift truck, the single largest energy drain on the battery is from the hydraulic system which is utilized for raising and tilting of the lift mast of the truck and for steering.
In the past, lift trucks have commonly used two or more hydraulic pumps for supplying the fluid pressure of the hydraulic systems. In some instances, such pumps are provided in separate and independent hydraulic circuits, one supplying fluid pressure to the lift cylinder, and another supplying pressure to the tilt cylinder or cylinders. If equipped with power steering, a third pump is used to independently supply fluid pressure to the steering system. In other instances, pumps of different sizes, for example, one large pump and one small pump have been utilized to supply fluid pressure to a common circuit.
One of the greatest deficiencies of such prior systems is the improper matching of the particular flow requirements of the hydraulic system with the output of their pumps. This is because the pumps are sized to meet the maximum flow requirements of the system and their outputs are dependent on the particular speed versus load characteristics of their electric drive motors and not the flow requirements of the system. As a result, the pump or pumps are normally generating more fluid flow than actually being required by the hydraulic system during the majority of operating situations. The excess fluid flow caused thereby is normally dumped back to tank without being used which wastes an inordinate amount of the limited energy available from the battery. This deficiency is somewhat reduced by the use of different size pumps mentioned above.
Another deficiency which robs an excessive amount of energy is the constant starting and stopping of the pumps which, due to the high inertial forces involved, places high peak energy loads on the battery. In the prior systems this occurs each time the mast is lifted or tilted or a steering correction is made. Steering is the greatest offender of this because of the large number of minor steering corrections made during operation.
The use of plural pumps and their associated electric drive motors and related components also greatly increases the cost and complexity of such prior hydraulic systems.